Last year, researchers made a game-changing realization: brown fat, the energy-burning stuff that keeps babies warm, isn't just for the youngest among us. Adults have it, too (if they are lucky, anyway), and it is beginning to look like the heat-generating tissue might hold considerable metabolic importance for familiar and irritating trends, like our tendency to put on extra weight as we age. If we can find a way to hold onto, make more, or activate brown fat, it might be one way to help keep us slim, according to scientists who have written a series of minireviews appearing in a special April issue of Cell Metabolism, a Cell Press journal.

"It's a new metabolic world; we can now ask questions we wouldn't have considered even one year ago," says Jan Nedergaard of Stockholm University, who authored one of the five reviews.

Brown fat was once the preoccupation of a few researchers studying rodents and newborn mammals. "At times, their work was deemed more an exercise in scientific curiosity than an issue relevant to human health," write Cell Metabolism editors Nikla Emambokus and Charlotte Wang in an editorial.

That all changed when three papers in the New England Journal of Medicine showed that adults have brown fat cells in their necks, where, as Sven EnerbÃ¤ck of GÃ¶teborg University explains, it has the unique ability to safely dissipate chemical energy in the form of heat. When we spend a lot of time in the cold, the amount of brown fat we have goes up.